Patent GB-1005177-A (GB)
Title: Improvements in or relating to the control of nuclear reactors
Inventor: TAYLOR ANTHONY JAMES
IPC: G21C7/12
Filed: 1961-12-22
Abstract: 1,005,177. Controlling nuclear reactors. BABCOCK & WILCOX Ltd. Dec. 21, 1962 [Dec. 22, 1961], No. 45986/61. Heading G6C. A nuclear reactor is controlled by means of conduits which extend through the core, each conveying a fluid at a velocity which varies along the length of the conduit and containing a body or bodies of neutron absorbent material movable along the conduit against a biasing force under the influence of the fluid flow, the positioning of the body or bodies in relation to the core being variable by alteration of the rate of fluid flow through the conduit. The watercooled and moderated reactor described by way of example has fuel elements each comprising a spaced array of fuel pins 4 within a casing of square cross-section through which water flows. In each fuel element, several of the positions of the array are occupied by tubes 25 which in one embodiment (Fig. 3) are formed of sections, the internal diameter of which increases progressively from the lower to the upper end of the tube and which are joined by shoulders 26. Each tube 25 is closed at its lower end but has an annular row of apertures 27 near this end while its upper end is connected by a tubular conduit 28 to a header 29 which receives the conduits 28 from all the tubes 25 of the fuel element. A pipe 30 leads from the header 29 to an adjustable valve (not shown) for controlling the outflow of water. Each tube 25 contains a plurality of neutron absorbing balls 33 formed of boron stainless steel with an outer steel coating. When the water flow rate through the tube is less than a critical value, the lowermost ball will (as shown) be in contact with an annular row of fins 34 near the lower end of the tube, but when it is above this value and below a second critical value, the column of balls 33 will be urged upwardly until the lowermost ball is in the neighbourhood of the lower annular shoulder 26. By continually increasing the water flow rate beyond successively higher critical values, the ball column is raised to successively higher positions in which the number of balls raised out of the core and into the conduit 28 is successively greater. The water flow rate may be varied over a range about a mean between two successive critical values without changing the position of the ball column. If all the valves in the water outflow pipes 30 are closed, or the pressure within the reactor pressure vessel fails, the balls 33 sink in the water under gravity to their lowermost positions in which the core is subcritical. The tubes 25 may be provided with internal longitudinally extending ribs to prevent contact of the balls 33 with the walls of the tube. In a modification, the tubes have internal diameters which have a continuous increase upwards so that the ball positioning can be adjusted more precisely but precise water flow rates are then required. In an alternative embodiment (Fig. 5) the tubes 25<SP>1</SP> are of uniform internal diameter with an annular row of apertures 35 in the tube wall below the fins 34 and annular rows of bleed apertures 41 at spaced levels along the length of the tube. When water is withdrawn from the top of the tube, water enters through all the apertures 35, 41 and flows upwardly within the tube so that the flow rate in any tube length between adjacent rows of apertures 41 is greater the greater the number of apertures 41 below this tube length and greater the greater the rate of water outflow from the tube. As the water outflow rate from the tube is increased, first the balls 33 above the uppermost row of apertures 41 are swept out of the tube 25<SP>1</SP> and then the balls 33 above the next uppermost row of apertures 41 and so on as the water outflow rate increases through successive critical values. A core having both kinds of tunes 25, 25<SP>1</SP> may be controlled so that when neutron absorbent material is removed from the core, that remaining is adjacent to and symmetrically disposed above and below a horizontal plane through the core centre. During operation in this way, when, during the early part of the core life it is not necessary to remove more than a small part of the neutron absorbent material, that remaining is adjacent the central horizontal plane of the core where it acts to render the thermal neutron flux distribution more uniform in the core axial direction; when, during the later part of the core life it is necessary to remove the greater part of the neutron absorbent material, that remaining is only at the top and bottom of the core and there is none adjacent the central horizontal plane where lie those parts of the fuel pins most depleted of fissile content. In the above embodiments, the balls 33 may be replaced by rods; in the case of the first embodiment (Fig. 3) the balls 33 in each tube 25 are replaced by a single rod, while in the second embodiment (Fig. 5) the balls 33 in each tube 25<SP>1</SP> are replaced by a plurality of rods arranged end to end, each of such rods replacing a group of balls sufficient in number to occupy a length of the tube 25<SP>1</SP> between adjacent rows of apertures 41 in the tube wall. Furthermore, the outflow pipes 30 may be arranged for simultaneous connection to a source of water at a higher pressure than in the pressure vessel so that a temporary reverse water flow is set up to hasten the fall of the balls or rods in the case of an emergency shut-down of the reactor.